Hydrocarbon (HC) emissions are a well-known and persistent threat to the environment. Commonly emitted HC species include precursors to smog and agents that are acutely toxic to human, animal and plant life. Moreover, the U.S. Environmental Protection Agency (EPA) has reported that automobile sources contributed 44% of the national emissions inventory of volatile organic compounds (VOC) in 2002. U.S. Environmental Protection Agency Clearinghouse for Inventories & Emissions Factors, “Air Pollutant Emission Trends: 1970-2002 Average Annual Emissions, All Criteria Pollutants” (January 2005). As defined by the EPA, a volatile organic compound is “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.” 40 C.F.R. §51.100. Included in this definition are many smog-forming hydrocarbon species.
VOC emissions from modern vehicles are primarily the result of the incomplete combustion of fuel (tailpipe emissions) and the evaporation of fuel stored on-board (evaporative emissions). Tailpipe emissions are highest immediately after starting. At moderate temperatures (20-30° C.), only 10-30% of gasoline vaporizes to join the combustible fuel/air mixture. As liquid fuel does not burn, this necessitates generous over-fueling to provide robust starting. Much of the excess fuel escapes complete combustion and encounters the catalytic converter, which is very inefficient at operating temperatures below approximately 300-350° C. The period of highest tailpipe HC emissions coincides with the period of lowest catalyst conversion efficiency. Consequently, a high percentage of HC emissions occur during the starting and warm-up periods. In fact, between 60 and 95% of all tailpipe emissions occur during the cold-start period, which includes the first 60-90 seconds of engine operation after starting the vehicle. Ashford, M. D., and Matthews, R. D. “Further Development of an On-Board Distillation System for Generating a Highly Volatile Cold-Start Fuel,” SI Combustion and Direct Injection SI Engine Technology, SP-1972, pp 161-167, Society of Automotive Engineers, Warrendale, Pa. (2005), the entire contents of which are incorporated herein by reference.
Research has demonstrated that use of a highly volatile fuel for cold-starting significantly reduces tailpipe HC emissions. See Kidokoro, T., Hoshi, K., Hiraku, K., Satoya, K., Watanabe, T., Fujiwara, T., and Suzuki, H., “Development of PZEV Exhaust Emission Control System,” SAE 2003-01-0817, the entire contents of which is incorporated herein by reference. The vapor inside the fuel tank primarily contains the lightest and most volatile species present in gasoline. See Lyons, J., Lee, J., Heirigs, P., McClement, D., and Welstand, S., “Evaporative Emissions from Late-Model In-Use Vehicles,” SAE Paper No. 2000-01-2958, the entire contents of which are incorporated herein by reference. In one study, speciation of vapor above liquid gasoline at 21° C. revealed that the vapor was dominated by three species—2-methylpropane (isobutane), n-butane, and 2-methylbutane (iso-pentane) collectively accounted for 78% of the vapor composition. See Siegl, W., Guenther, M. and Henney, T., “Identifying Sources of Evaporative Emissions—Using Hydrocarbon Profiles to Identify Emission Sources,” SAE Paper No. 2000-01-1139, the entire contents of which are incorporated herein by reference. In one study, an on-board fuel preprocessor that collected highly volatile fractions of gasoline for use as starting fuel was successful at reducing overall tailpipe HC emissions by more than 80%. See Ashford, M. D., and Matthews, R. D. “Further Development of an On-Board Distillation System for Generating a Highly Volatile Cold-Start Fuel,” SI Combustion and Direct Injection SI Engine Technology, SP-1972, pp 161-167, Society of Automotive Engineers, Warrendale, Pa. (2005). Further studies and research have demonstrated that the most desirable starting fuels are rich in HC species no heavier than C6. See Stanglmaier, R., Roberts, C., Ezekoye, O. and Matthews, R., “Condensation of Fuel on Combustion Chamber Surfaces as a Mechanism for Increased HC Emissions During Cold Start,” SAE Paper No. 1997-01-2884, the entire contents of which are incorporated herein by reference. Thus, vapor from a fuel tank is an ideal source for a starting fuel that is rich in light-weight and highly volatile HC species.
Although tailpipe emissions are substantial, evaporative HC emissions can be three to four times greater than tailpipe emissions during routine driving. Lyons, J., Lee, J., Heirigs, P., McClement, D., and Welstand, S., “Evaporative Emissions from Late-Model In-Use Vehicles,” SAE Paper No. 2000-01-2958. Evaporative emissions can be broken down into several classifications, with running losses and refueling losses accounting for the vast majority of total evaporative emissions. Refueling losses occur when vapor is displaced by liquid entering the fuel tank, a common occurrence at service stations. Running losses occur when vapor is generated in response to heat from hot exhaust, hot pavement, a hot engine compartment, or another hot component of a running engine. Most fuel systems are “return” style systems that supply a large amount of fuel to injectors while returning the excess fuel to the fuel tank. This excess fuel helps keep engine-bay fuel temperatures low, reducing vapor generation and the chance of vapor lock. However, the returning fuel is warmed during its trip through the engine-bay. Also, rear mounted fuel tanks are located in close proximity to hot exhaust pipes, which can be a potent heat source, especially in stop and go traffic. Thus, even though modern fuel return systems help to reduce the amount of vapor generation, a substantial amount of vapor continues to be generated due to heat sources surrounding the fuel tank.
Other classifications of evaporative emissions include diurnal emissions and hot soaks. Diurnal emissions occur when vapor is generated in response to daytime increases in ambient temperature. For example, gasoline tanks may receive considerable heat input via radiation from a hot pavement. In addition, hot soaks occur when vapor is generated due to high temperatures that result from a lack of circulating air or engine coolant after shutdown. Following shutdown, vehicles actually tend to warm up somewhat (especially under the hood) because of a loss of the cooling effect of air flow and water circulation. Naturally, the fuel system is warmed, resulting in the hot soak emissions. These hot soak vapors can become trapped in the fuel system and cause fuel system vapor lock during hot restarts.
The industry-standard solution for preventing the atmospheric release of excess fuel vapor is to collect the vapors in a canister filled with an activated charcoal adsorbent before combusting the vapors when engine operating conditions are favorable. This related art system is shown in FIG. 1. This is an imperfect solution for several reasons. First, diurnal, refueling and hot soak emissions occur when the engine is off. Therefore, when the carbon adsorbent becomes saturated (e.g., several consecutive diurnal cycles with no canister purge), vapors will be released to the atmosphere (canister breakthrough). Second, running loss emissions result when vapor is generated at a very high rate (such as during sloshing), exceeding the adsorption capability of a typical passive vapor-adsorbing system. Finally, the desorbed vapors inducted by the engine include a considerable—but unknown—quantity of air. This fuel/air mixture of unknown strength can upset the delicate air/fuel balance necessary to provide low-emission combustion and smooth engine operation. For this reason, the vapor canister is seldom purged during starting or at idle, where the consequences of engine upsets are most severe.
All of the aforementioned factors ultimately result in higher HC emissions. However, several approaches to reducing evaporative emissions exist. So called “returnless” fuel systems reduce running losses by eliminating the return of warm fuel from the engine bay. Newer fuel tanks are vented through the filler neck such that fuel vapors can be recovered by specially equipped fuel station pumps, reducing refueling losses. The newest, cleanest cars in the world (California PZEV-level) are certified to generate zero fuel based evaporative emissions. These vehicles combine returnless fuel systems with highly adsorbent carbon canisters.
Nevertheless, evaporative emissions still represent a significant portion of total vehicular VOC emissions. Despite a decrease in allowable tail pipe emissions by three orders of magnitude over the last thirty years, actual HC emissions have decreased by approximately 80% on a car-by-car basis. Factoring in the increase in vehicle miles traveled over the same period, the aggregate mobile-source HC emissions have only decreased by about half. Furthermore, older vehicles that are not subject to regulations issued after the date of their manufacture may have disproportionately higher emissions as compared to newer vehicles.
It is known that passive starting systems can be incorporated into automobiles to induct hydrocarbon vapors from the carbon canister. A major concern with these systems is the limited production of fuel vapor in cold weather conditions. Under conditions where ambient temperatures are low and there is limited thermal driving force for fuel evaporation, such as, for example, during winter months, the production of fuel vapor is diminished. However, volatile species are added to gasoline in cold climates to enhance cold-start performance, and this increased volatility compensates for the decrease in fuel vapor. However, these systems also suffer from fundamental imperfections. First, there is no way to conclusively know the amount of vapor that can be drawn from the canister. Second, the vapors inducted from the carbon canister are mixed with air, creating a mixture of unknown strength. Thus, these systems are unsuitable for cold-starting, when predictable and robust fueling is essential.
Active recovery of fuel vapor is believed to allow for simultaneous reductions in tailpipe and evaporative emissions. As previously discussed, use of the highly volatile components of fuel vapor as a starting fuel can greatly reduce tailpipe emissions. Thus, it is believed that an effective starting fuel for this purpose can be composed of condensed fuel vapor, which can be stored in isolation and include the most volatile HC species. In addition to the condensate formed from fuel vapor, active vapor recovery can also generate air and trace hydrocarbons that can be stored in a typical vapor canister. It is believed that the separation of the vapor from air will greatly reduce the amount of evaporative hydrocarbon emissions when compared to a normal fuel system.
Given the increasing prevalence of alternative fuels, modern fuel systems and internal combustion engines that are capable of operating with multiple starting fuels are believed to be preferable to fuel systems that are designed to operate with only a single starting fuel. Thus, it is believed that a modern fuel system and internal combustion engine should preferably have multiple operating modes that are adjusted in response to the sensed composition of fuel within the fuel supply system. These multiple operating modes are believed to allow for optimized engine performance regardless of the composition of the fuel used to operate the engine.
Accordingly, it is desirable in the pertinent art to provide an on-board vapor recovery system for use with a fuel supply system and an internal combustion engine that addresses the limitations associated with known systems, including but not limited to those limitations discussed above. Specifically, it is desirable in the pertinent art to provide a vapor-recovery system that can capture, store, and use isolated fuel vapors as a highly volatile fuel source. In addition, it is desirable in the pertinent art to provide an engine control unit for use with a fuel supply system and an internal combustion engine that can select the fuel source and desired operating mode of the internal combustion engine based on the composition of fuel and/or vapor at different locations within the fuel supply system.